1. Technical Field
The present invention relates in general to the field of data processing systems and more specifically, to the field of storage media within data processing systems. Still more specifically, the present invention relates to an improved system and method for improving the reliability of storage media within data processing systems.
2. Description of the Related Art
Incorporated in modern data processing systems (e.g., a computer system) are typically storage media implemented as a hard disk drive. Each hard disk drive includes a servo mechanism for read/write head positioning and a disk medium for data storage. The read/write head, herein referred to simply as a xe2x80x9cheadxe2x80x9d, reads a set of position information stored on the disk medium to detect the current head position above the disk medium. This set of position information is written on the disk medium in a radial direction and is periodically detected in accordance with a rotation accuracy of the disk medium.
Each set of position information usually includes a servo address mark (SAM), a Gray code, and burst data. The head reads the SAM periodically to trigger a servo locked state. Generally, the address information in a Gray code following the SAM is read in the servo locked state, so that a track position (a cylinder position) of the head is read.
An actuator, implemented as a voice coil motor (VCM) controls the movement of the head arm. The trajectory of the head is calculated from the current address read by the head and a target address. The head follows the track to move to the target address. Generally, because a hard disk drive is designed for maximum performance and a minimum seek time, the maximum velocity of the head reaches 3.3 m/sec during a fast seek operation.
In recent years, the recording density of hard disk drives has been increased. However, this increase in recording density results in a greater occurrence of errors during a position information read. Some factors that contribute to this increase of errors include: stresses caused by ambient conditions (e.g., temperature or a power source), imperfectly written position information, a momentary fault of the head, or a variety of other factors. It is difficult to eliminate every one of these error factors. An error recovery method incorporated into the system can help alleviate some of the error factors. This is especially true for a ramp loading/unloading mechanism recently employed in hard disk drives to improve the impact properties and to prevent the wear of the head. In the ramp loading/unloading mechanism, the head encounters the ramp at a standby position and the head is positioned higher than when the head moves above the disk medium. Consequently, when the head overruns the ramp at a rapid velocity (e.g., at least 1 m/s) due to a seek error, the head collides against the crush stop mechanism positioned at the outer periphery of the ramp. The head rebounds, possibly causing the surface of the disk medium to be damaged.
In conventional hard disk drives, when the head fails to read a SAM, the servo locked state is reset to the servo unlocked state. The VCM output is set in the neutral state to retry the SAM read. Because the VCM is in the neutral state, the head arm continues to move due to its inertia. And, when a Gray code read fails, a state estimator detects Gray code read failure. The state estimator records actual values measured from the disk media and compares the actual value to an estimated value. The state estimator also corrects an estimated value utilizing a difference (estimation error) between a measured value (read Gray code) and the estimated value. The output (an estimated value) of the estimator in the stable state offers enough precision for this application. If a difference from a measured value is greater than a predetermined reference value, the estimated value is utilized. If a Gray code read error is detected, the error may be corrected utilizing the estimated value.
Japanese Published Unexamined Patent Application No. 9-139034 discloses a recovery technique, utilized when a servo error occurs during a fast seek operation. According to the technique disclosed in the official gazette, the driving current is stopped immediately if an estimated velocity exceeds a predetermined reference value and a braking current is applied according to the difference between the estimated velocity and the reference velocity to reduce the head velocity to at least a predetermined safety value before the head collides against the stopper.
Also, Japanese Published Unexamined Patent Application No. 11-45522 discloses a technique for filtering the driving current through an integrator, which generates an integrated deceleration control signal. Integrating the driving current begins at a seek operation when a seek error is detected.
Furthermore, Japanese Published Unexamined Patent Application No. 6-187749 discloses a technique for controlling deceleration of a head of a hard disk drive by changing a normal velocity control signal to a deceleration signal recorded beforehand in a memory circuit when a seek error occurs.
Finally, Japanese Published Unexamined Patent Application No. 4-106768 discloses a technique for controlling a decelerated velocity utilizing a normal time velocity table at a normal time and an abnormal time velocity table at an abnormal time.
However, any of the conventional techniques described above cannot solve such seek errors satisfactorily. The technique that neutralizes the VCM, is not effective when the inertial velocity is over 1 m/sec. If a SAM cannot be read when the VCM is neutralized, permanent damage to the surface of the disk medium may result. In addition, if the SAM is read, it is not certain that the head velocity can be decreased enough in the remaining area.
If a Gray code is misread, the use of the above described state estimator will be effective when the error rate is low. However, the error rate rises unavoidably if an attempt is done to record Gray codes at a higher density, and thereby, the state estimator does not function adequately. If the position of the estimator deviates from an actual position of the head, the use of an estimated velocity value might accelerate the head unexpectedly. In addition, if the estimator determines the actual value read from the disk medium is correct, the head velocity is sometimes accelerated unexpectedly. Such unexpected increase of the head velocity might cause the head to overrun and may result in the generation of abnormal sounds or vibration due to discontinuous VCM outputs.
To overcome the foregoing and additional limitations of the prior art, the present invention presents an improved system and method for implementing a rotary memory device. The method of the present invention enables a dummy SAM to be generated at a rate that does not reset the servo locked state even when the process of detecting a SAM fails. Consequently, it is possible to control servo interruptions periodically, to attempt to return the head to the normal reading state. Whether or not the SAM is read can be determined by checking whether or not a SAM read signal (smf) is output in a time window (SAM coming window) in which the SAM should be read. If a dummy SAM is generated, a dummy sector interval is utilized as a sector interval to prevent the delay of the next servo gate.
Servo interruptions can thus be controlled normally by generating a dummy SAM even when a SAM fails to be read by the head. If the normal state cannot be restored within a fixed number of intervals, the system goes into the emergency brake control mode. In this emergency brake control mode, the direction of the head is determined, and the VCM current is to drive the arm in the opposite direction. The driving current value and the driving time are predetermined. The output value from the back electromotive force (emf) circuit utilized for the loading/unloading mechanism can control the servo interruptions with feedback control. Damage to the disk medium and subsequent data loss problems are reduced because the velocity of the head is reduced to a safe level.
Even during the emergency brake control mode, attempts are made to read a SAM and corresponding Gray code. If a SAM is detected during the emergency brake control mode, the head can be returned to the normal control mode.
Furthermore, the present invention proposes a method to recover from a Gray code misread. When a Gray code is misread, the head accelerates at a rate that is not expected from the last track position and an unexpected acceleration input value is relayed to the servo controller. Or, the head reads a value much greater or much less than a value estimated by the state estimator. The unexpected values are replaced with the predetermined maximum values, so the servo interruptions are controlled. If an unexpected value utilized for an extended period of time, the system can enter the emergency brake control mode. It is also possible to prevent the hazard counter from being reset when a misread occurs. If a count recorded by the hazard counter exceeds a predetermined safety value, the system enters the emergency brake control mode.
Each of those methods to recover from a Gray code misreading is intended to replace a discontinued current input (electric power application) to the VCM with an input value that can suppress generation of an abnormal sound/vibration from the VCM.